Mechanistic and bioenergetic aspects of the interaction between blood group antigens and their specific antibodies are studied using radioiodinated and enzyme-labeled antibodies in conjunction with sensitive continuous flow Auto Analyzer assays of antigenic expression and antibody activity. Present studies are specifically concerned with the relationship of surface charge to agglutination, thermodynamic measurements of antibody attachment at the cell membrane and improved methodology for the standardization of therapeutic reagents. Other areas of interest are mapping antigenic sites on the red cell membrane and defining the major macromolecular components of antigens other than ABH and Lewis.